This application is based upon and claims the benefit of priority from Japanese Patent Application No. 213587/2000, filed Jul. 14, 2000, the entire contents of this application are incorporated herein by reference.
This invention relates to the fabrication of the lasers described below, as well as their use in various applications including micromachining, analyses by high-intensity short-pulse x-rays and medical diagnoses and treatments.
(1) Fabrication of Industrially Applicable Short-pulse High-peak Power Lasers
Short-pulse high-peak power lasers have a history of more than 10 years on the commercial market and yet they have not been used at industrial sites. FIG. 10A shows a conventional laser system constructed by incorporating a Ti:sapphire laser into the CPA (chirped pulse amplification) technology; as shown, the system consists of a mode-locked laser (and a pump laser), a diffraction grating paired pulse stretcher, a regenerative amplifier (and a pump laser), a master amplifier (and a pump laser), and a diffraction grating paired pulse compressor. The system can produce a peak power as high as several tens of terawatts but the repetition rate is only about 10 Hz. Other disadvantages of the system include bulkiness, high cost, difficult adjustments and low reliability for consistent use over a prolonged period.
FIG. 10B shows a Yb-fiber laser which consists of a fiber mode-locked laser, a fiber pulse stretcher, a fiber amplifier and a fiber pulse compressor. Although this system is compact and has repetition rates on the order of MHz, the limitation of fiber resistance to intense light makes it difficult for the system to output power in excess of mJ per pulse which is a minimum requirement for industrial use.
In order to solve these problems, the present inventors took two approaches. First, in order to shorten the pulse duration, the diffraction grating which was bulky, difficult to adjust and costly was replaced by SBS (stimulated Brillouin scattering) cells (or crystals) and nonlinear Raman crystals; second, the Ti-sapphire crystal was replaced by a Raman cell amplifier to reduce thermal load and enhance reliability.
The new systems are compared with the conventional systems in the following table, in which the problems with the conventional systems are labelled with dots and the features of the new systems with open circles.
(2) Various Uses of the Invention
a) non-thermal fine-machining: as a light source for precision machining of semiconductors and marking of electronic-grade glass
b) microscopes of exotic function: as a light source for multi-photon microscope
c) lithography: as a light source for USLI fabrication
d) x-ray fluorescence spectroscopy: enables analysis of elements in ultra-low levels
e) high intensity x-ray nondestructive analyzers: as a light source enabling the measurement of low radiation doses
f) short-pulse x-ray diffractometer: as a light source for measuring ultra-fast structural changes
g) dental x-ray imaging apparatus: as a light source enabling imaging at low radiation dose
h) dental scale removers: as a light source for selective removal of scale without adverse effects on the enamel
i) high-precision x-ray imaging apparatus: as a light source for projecting high-resolution x-rays
j) sterilizing apparatus: as a light source for noninvasive local sterilization
k) apparatus for removing or transplanting hair: as a light source for painless surgery
l) removal of fouling films, oxide films, plates and paints: no damage to surfaces
To produce high-peak power short-pulse laser beams, the following CPA-based methods have been used but they have several problems from a practical viewpoint.
(1) Fiber Chirping Method
In this method, laser oscillation is performed with ordinary lasers and the generated pulses are compressed by eliminating chirping over fibers; the method is implemented by a Nd:YAG laser, Nd:glass laser, Nd:YLF laser, etc. Alternatively, a fiber laser is directly oscillated and the generated pulses are compressed over fibers; this method may be implemented by a Yb fiber laser which employs Yb glass. Whichever method is used, high-intensity light is passed through an extremely thin fiber, so in order to avoid optical damage, the laser power is limited and the pulse energy is no more than about 1 mJ.
(2) Grating Chirping Method
a) Ti:sapphire Laser (see FIG. 10A)
This is a typical CPA-based laser and the technique which uses a pair of diffraction gratings is quite common. However, the diffraction gratings require not only a wide installation space but also precise adjustments, so the technique is too costly to be useful in general industries. In addition, the mode-locked laser as an oscillator and the regenerative amplifier also require precise adjustments. As a further problem, when a high average power is produced, the lasing crystal gives off heat in an amount corresponding to quantum efficiency and the thermal distortion from the heat generated in the crystal must be taken into account.
b) Optical Parametric Short-pulse Laser (OPCPA, or Optical Parametric CPA)
This laser performs amplification based on nonlinear effect, so the thermal load imposed on it is much smaller than what is experienced by the Ti:sapphire laser. However, it still needs a mode-locked laser as an oscillator, a pulse stretcher using a diffraction grating pair, and a pulse compressor. Furthermore, the conditions for optical parametric amplification are so rigorous that efficient light emission is not easy to realize.
To solve these problems, the following techniques are used in the present invention.
(1) In place of the bulky diffraction grating pair, a very small nonlinear crystal is used to compress pulses. In extending and compressing pulses, precise adjustments are necessary but this requires considerable space and adds to the cost. Therefore, to minimize the required space, pulses should be compressed within a very small crystal.
(2) High power far in excess of the limit on fiber output is produced. Use of fibers in place of the diffraction grating pair has been known for years but high power cannot be produced since the passage of light through a thin fiber can cause optical damage. To cope with this problem, the present inventors developed an optical system capable of generating short pulses in high power by making use of the stimulated Raman scattering in crystal.
The Raman compression technology is known but the Raman light to be amplified usually has power with an extremely low noise level, so it has been necessary to use extremely intense pump light but then the threshold for the generation of Raman light is high enough to increase the chance of optical damage. To avoid this problem, particularly in the case of using crystals having high thresholds for the generation of Raman light, the present inventors adopted the half-waist reflection method and the tandem crystal method. In the half-waist reflection method, the crystal is so cut that the emerging laser beam has a half waist at the exit face and the crystal end face is either reflection coated or fitted with a mirror to collect the scattered light, thereby lowering the threshold level. In the tandem crystal method, two crystals are used, one as an oscillator to generate faint Raman light and the other for amplifying it; as the result, the threshold for the generation of Raman light is significantly lowered and, at the same time, the beam fluctuations are minimized by saturation amplification.
(3) High-power short-pulse pump light is produced. To produce short enough pulses by Raman scattering, the development of a short-pulse high-power pump laser is essential. To this end, the present inventors propose a SBS-based short-pulse pump laser system which uses a tandem SBS cell method, or two different SBS cells.
In order to ensure consistent generation of intense short-pulse light from one SBS cell, simultaneously occurring SRS must be prevented from circulating through the system. To meet this need, the generated SBS light is amplified by passage through the other SBS cell. Because the two SBS cells are made of different materials, the SRS generated in the first cell is not amplified in the second cell.
(4) Amplification is not done by the lasing medium but by a Raman crystal having a much smaller thermal load.
The biggest problem with high-power lasers is thermal load. The Ti:sapphire laser and other conventional lasers suffer loss due to quantum efficiency and this loss propagates through the crystal as heat, making it difficult to produce high power. In Raman amplification, short-pulse Raman light can be amplified efficiently without heat generation and hence high-power beams can be produced without deterioration in beam quality.
(5) Non-CPA-based multi-stage compression
In CPA, the pulses generated by the mode-locked laser are first stretched, then amplified and finally compressed. This involves a lot of waste. The system of the invention starts with pulses having a duration of several hundred picoseconds, which is gradually decreased to sub-picoseconds through multiple stages, thereby achieving high efficiency and compactness. In the multi-stage approach, compression can be accomplished to a time range equivalent to the phonon life and the optical characteristics of nonlinear crystals must first be understood thoroughly.
The present invention has the following objectives.
(1) To realize a compact, high-pulse-energy power and short-pulse laser using an amplification system that is theoretically free from heat generation.
(2) To perform automatic correction of wave planes by generating Raman light in the presence of thermal distortion.
(3) To realize a compact system that can produce shorter pulses without using the pulse extender, pulse compressor and other devices used in CPA that are bulky and which require precise adjustments.
To attain these objects, the present inventors developed the following techniques.
a) Compressing sub-nanosecond laser pulses by the tandem SBS method using the stimulated Brillouin scattering effect.
b) Multi-stage compression of pulses to a duration as short as the phonon life (sub-picoseconds) using the stimulated Raman scattering effect.
c) Introducing seed pulses by the half-waist reflection method and the tandem crystal method for the purpose of reducing the simulated Raman scattering effect.
The development of these techniques requires optical data for nonlinear crystals but none of them have been available. Hence, the present inventors found by experiment the limit on optical damage and the threshold intensity of the pump laser for the generation of optical scattering in a laser field that was intense but not so strong as to cause channeling due to self-focusing.
Thus, the present invention provides a short-pulse pump laser light generator consisting essentially of:
1) a solid laser pumping mechanism which does not use lossy fibers but which forms parallel beams from a semiconductor laser by means of a cylindrical telescope consisting of cylindrical lenses and reshaping prism optics comprising combined prisms, with the parallel beams being condensed with a collimator lens to produce high-intensity light that can pump the active medium in the solid laser;
2) a pumped laser light oscillator incorporating said mechanism 1);
3) a laser light amplifier incorporating said mechanism 1); and
4) a SBS pulse compressor which comprises condensing optical element cells made of two different materials and which generates short-pulse laser light by the stimulated Brillouin scattering (SBS) effect.
In the generator, the pumped laser light from the oscillator is introduced into a preamplifier and amplified; the amplified light is introduced into the SBS pulse compressor and the resulting short-pulse laser light is introduced into the same preamplifier and amplified; the amplified laser light is introduced into another preamplifier, amplified and further amplified in a master amplifier, thereby producing amplified laser light having a shorter pulse duration.
The present invention also provides an ultra-short pulse amplified laser light generator consisting essentially of:
a) a mechanism for producing ultra-short pulse amplified laser light in which the short-pulse pump laser light generated by the apparatus described above is condensed by condensing optics, introduced into a Raman crystal and reflected with a reflector back into the Raman crystal to form phonon lattices;
b) a Raman light oscillator incorporating said mechanism a);
c) a laser light amplifier/compressor incorporating said mechanism a).
In this generator, the short-pulse pump laser light is supplied into the pump laser light amplifier and amplified; then, the Raman oscillator is pumped and the emitted light is introduced into the Raman amplifier, amplified and supplied into another Raman amplifier pumped with first Stokes radiation; the light is amplified in the second Raman amplifier, from which it is output as ultra-short pulse amplified Raman laser light.
The invention also provides:
d) an apparatus that incorporates the generator and which not only generates harmonics but also outputs compressed pulses.